In an attempt to improve the gas mileage, automotive engineers are making efforts to reduce car weight. One example of their efforts is to fabricate rocker arms with carbon fiber reinforced plastics (CFRP) in place of cast iron. Some of the inventors of the present invention have proposed a lightweight CFRP-made rocker arm in Japanese Utility Model Application (OPI) No. 103610/81 (the symbol OPI means an unexamined published Japanese patent or utility model application).
A rocker arm comprises two sides, one being connected to an engine valve by an adjusting screw and the other side communicates with a cam and a hole through which a rocker shaft is inserted. The first side is hereunder referred to as the valve side and the other side as the cam side. The valve side has an area on which the adjusting valve is mounted, and the cam side has a cam contact face. An example of the conventional rocker arm is illustrated in FIG. 1. During service, the rocker arm pivots on the rocker shaft and loads are applied at the adjusting screw mounting area and at the cam contact face.
In Japanese Utility Model Application (OPI) No. 103610/81, the rocker arm is made of a laminated sheet of prepregs wherein the fibers are oriented at an angle of .+-.45 degrees, and which are arranged in a direction perpendicular to the direction of stress application, or parallel to the axis of the rocker shaft. The rocker arm proposed in this reference has the desired strength and among other things, it is light weight and can be used advantageously in engines. However, the present inventors have found that the strength of this rocker arm is by no means sufficient for use in a high-speed engine because cracks sometimes developed at the interface between each prepreg when the engine was run under heavy load (i.e. at high speeds). This problem could not be completely solved by orienting fibers at different angles in two adjacent prepregs, or by changing the strength of carbon fibers or the proportion of carbon fibers in the plastics.